Glass articles are extensively found in homes, offices, and factories in the form of lenses, prisms, mirrors, CRT screens, and other items. Many of these glass surfaces are used with optical components which require that the surface be optically clear and have no visible defects and/or imperfections. If present, defects, imperfections, and even minute scratches may inhibit the optical clarity of the glass article. In some instances, these defects, imperfections, and/or minute scratches may inhibit the ability to accurately see through the glass. Glass surfaces used with optical components must be essentially free of any defect, imperfection, and/or scratch.
Many glass surfaces are curved or contain a radius associated therewith. These radii and curves are generally generated in the glass forming process. However, as a result of the glass forming process, defects such as mold lines, rough surfaces, small points, and other small imperfections may be present on the outer surface of the glass. These defects and/or imperfections, however small, tend to affect the optical clarity of the glass. Abrasive finishing processes have been widely used to remove such imperfections and/or defects. The abrasive finishing typically falls within three main processes: grinding, fining, and polishing.
Glass finishing is typically done with a loose abrasive slurry. The loose abrasive slurry comprises a plurality of abrasive particles dispersed in a liquid medium such as water. The most common abrasive particles used for loose slurries are pumice, silicon carbide, aluminum oxide, garnet, and the like. The loose abrasive slurry may optionally contain other additives such as dispersants, lubricants, defoamers, and the like. In most instances, the loose abrasive slurry is pumped between the glass surface that is being finished and a lap pad, such that the loose abrasive slurry is present between the glass surface and the lap pad. The lap pad may be made from any material such as rubber, foam, polymeric material, metal, steel, and the like. Typically, both the glass workpiece and the lap pad will rotate relative to each other. This process typically comprises one or more steps, with each step generating a progressively finer surface finish on the glass.
Rough grinding steps perfect the desired curve or radius and remove any casting defects by rough grinding the glass surface with an abrasive system that includes a metal-button lap used with a rough slurry of aluminum oxide or garnet. However, the abrasive tool in this rough grinding process will impart coarse scratches into the glass surface such that resulting glass surface is neither precise enough nor smooth enough to directly polish to an optically clear state. The objective of the grinding process is to remove large amounts of glass quickly and fairly accurately while leaving as fine of a scratch pattern as feasible. These scratches are then typically removed by further steps commonly known as “fining” and “polishing”, which use finer slurries and softer pads.
The roughness of a surface is typically due to scratches or a scratch pattern, which may or may not be visible to the naked eye. A scratch pattern may be defined as a series of peaks and valleys along the surface. Rtm and Ra are common measures of roughness used in the abrasives industry, however, the exact measuring procedure may vary with the type of equipment utilized in surface roughness evaluation.
Ra is defined as an average roughness height value of an arithmetic average of the departures of the surface roughness profile from a mean line on the surface. Generally, the lower the Ra value, the smoother the finish. Measurements are taken at points both above and below the mean line on the surface within an assessment length set by the measurement instrument. Ra and Rtm (defined below) are measured with a profilometer probe, which is a 5 micrometer radius diamond tipped stylus and the results are recorded in micrometers (μm). These departure measurements are totaled and then divided by the number of measurements to arrive at an average value.
Rt is defined as the maximum peak-to-valley height. Rtm is the average, measured over five consecutive assessment lengths, of the maximum peak-to-valley height in each assessment length. In general, the lower the Rtm value, the smoother the finish. A slight variation in the Ra and Rtm values may, but not necessarily, occur when the measurement on the same finished glass surface is performed on different brands of commercially available profilometers.
The final step of the overall finishing process is the polishing step which generates the smoother, optically clear surface on the glass article. In most instances, this polishing step is done with a loose abrasive slurry, since the loose slurry typically generates an optically clear surface that is essentially free of any defects, imperfections, and/or minute scratches. Typically, the loose abrasive slurry comprises ceria abrasive particles dispersed in water.
Although loose abrasive slurries are widely utilized in the fining and polishing steps to provide an optically clear surface finish on glass articles, loose abrasive slurries have many disadvantages associated with them. These disadvantages include the inconvenience of handling the required large volume of the slurry, the required agitation to prevent settling of the abrasive particles and to assure a uniform concentration of abrasive particles at the polishing interface, and the need for additional equipment to prepare, handle, and dispose of or recover and recycle the loose abrasive slurry. Additionally, the slurry itself must be periodically analyzed to assure its quality and dispersion stability which requires additional costly man hours. Furthermore, pump heads, valves, feed lines, grinding laps, and other parts of the slurry supply equipment which contact the loose abrasive slurry eventually show undesirable wear. Further, the steps which use the slurry are usually very untidy because the loose abrasive slurry, which is a viscous liquid, splatters easily and is difficult to contain.
Understandably, attempts have been made to replace the loose abrasive slurry finishing steps with lapping, coated, or fixed abrasive products. In general, a lapping abrasive comprises a backing having an abrasive coating comprising a plurality of abrasive particles dispersed in a binder. For example, U.S. Pat. Nos. 4,255,164; 4,576,612; 4,733,502; and European Patent Application No. 650,803 disclose various fixed abrasive articles and polishing processes. Other references that disclose fixed abrasive articles include U.S. Pat. Nos. 4,644,703; 4,773,920; and 5,014,468.
However, fixed abrasives have not completely replaced loose abrasive slurries. In some instances the fixed abrasives do not provide a surface which is optically clear and essentially free of defects, imperfections, and/or minute scratches. In other instances, the fixed abrasives require a longer time to polish the glass article, thereby making it more cost effective to use a loose abrasive slurry. Similarly in some instances, the life of a fixed abrasive is not sufficiently long to justify the higher cost associated with the fixed abrasive in comparison to loose abrasive slurries. Thus, in some instances, fixed abrasives are not as economically desirable as loose abrasive slurries.
What is desired by the glass industry is an abrasive article that does not exhibit the disadvantages associated with a loose abrasive slurry, but that is able to effectively and economically grind a glass surface in a reasonable time by providing fast stock removal.